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Superconductivity came with a lot of unfulfilled promises. Power without loss? Sign me up. Superconducting magnetic resonance imaging magnets? They're, ahem, cool. And CERN couldn’t operate without buckets of liquid helium to keep its magnets superconducting.

But those examples highlight the problem: pretty much all practical applications for superconductivity require liquid helium temperatures.The search for high-temperature superconductors has taken us to many weird places, including strange substances that only form at high pressure. Now we can add another of those substances to the list: a hydride that only forms under protest. Once formed, though, it may be a superconductor way above room temperature.

Further Reading

Why are we still looking for superconductors?

The search for superconductors goes on because current superconductors come with a number of challenges. If the magnetic field is too strong, superconductivity vanishes. Likewise, if the current density exceeds a certain limit, the resistance appears, which heats the conductor, leading to rapid—and rapidly expanding—failure. And the liquid helium needed to keep things cool in the first place is expensive.

Low-temperature metallic semiconductors are not the only game in town, though. There are certain ceramics that become superconductive at much higher temperatures. For these materials, you only need liquid nitrogen, which is much cheaper than liquid helium. There are even industrial processes for creating bendable superconducting wires from these ceramics.

But you won’t find power transmission lines made of superconducting ceramics, as maintaining a steady supply of liquid nitrogen is a real challenge. High-temperature superconductors haven’t captured the market in MRI magnets, either—the magnetic fields are too strong for them. Even though superconductivity might start at 77K, the magnet still has to be operated near 4K using liquid helium to withstand the effects of the magnetic field. So you might as well use a cheaper metallic wire and be done with it.

Enter hydrides

Hydrogen is a very simple element. But it's also weird. It should be a metal, but once solidified, it isn’t. It’s predicted to become metallic at high pressures—and maybe even become a superconductor. The observations for these states are still controversial, though. Hydrides, on the other hand, combine the best of both worlds. Metal atoms will often bond quite nicely to hydrogen. Depending on the materials and the hydrogenation state, a hydride may flip between conducting and non-conducting. If enough hydrogen is added, a hydride may even become superconducting.

The trick is getting the metal to accept being linked to all those extra hydrogens—as an example, we are talking about a lanthanum atom with 10 hydrogen atoms attached. To do that requires high pressure; otherwise, all the hydrogen escapes. Making a superconductor from a hydride is generally the worst of both worlds—to create your superconducting material, you need pressures in the area of hundreds of GigaPascals, and you also need temperatures somewhere between liquid nitrogen and liquid helium.

On the positive side, there are so many hydride combinations that we haven’t yet exhausted all the possibilities yet. To explore this space, a group of researchers took a lithium/magnesium alloy and started calculating how its properties change as hydrogen is added. They found several stable phases from LiMgH2 through to LiMgH16. Once the researchers had found all the potentially stable hydrides, they started examining their electronic properties.

The researchers were looking for the coupling between electrons and a type of sound wave, called a phonon. Superconductivity requires that electrons pair up, as the pairs then form a kind of extended object that can move without resistance. The pairing is communicated through phonons.

So to achieve superconductivity, electrons need to easily generate and absorb phonons. In LiMgH16, the researchers found a hydride with exceptionally strong coupling. When the researchers calculated the temperature at which LiMgH16 would superconduct, they found it to be a remarkable 350K (77°C). Unfortunately, LiMgH16 only happens to be stable at a pressure of about 250GPa.

It’s still a really interesting finding, because there are so many different hydride possibilities out there. Although 250GPa is too high for practical use—it may even be too high to allow for an experimental demonstration of superconductivity—it points toward the sorts of structures that are required to achieve superconductivity in hydrides. Other hydrides, maybe ones that involve three metals, might require lower pressures to be stable and may even superconduct at reasonable temperatures.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com

Was this based on any experimental evidence or is this being derived from pure theory? If it's the latter, what's stopping them from just running the same calculations on every possible hydride combination?

Yes, it's theoretical:

Quote:

Although 250GPa is too high for practical use—it may even be too high to allow for an experimental demonstration of superconductivity

As for "every possible hydride combination" - there are potentially a lot. Each additional metal atom you add will increase the space by a lot, there are 95 different 1-metal-atom combinations, 9025 2-metal-atom combinations, 857,375 3-metal....and so on. And it could be a case where the equations get significantly more difficult as you increase the number of components.

I assume they will continue exploring this route, but I'm guessing it's not trivial to check everything.

That is assuming 1:1 stoichiometries. Once you blow off that assumption, you start to get rather large numbers in a truly bigly way.

I think the way I did the math accounts for that that - i.e. I did powers of 95 (the number of metals), rather than 95*94*93... (i.e. I assumed duplicate atom types were allowed in the combinations)

So, for example, a "3-metal-atom" could be Mg2Fe. But then I also end up including, e.g., Mg3, and I don't know if that's distinct from just Mg or Mg2 in this case.

But I might be misunderstanding what you mean, chemistry (and it's subcategories like metallurgy) is not one of my strengths.

There are distinct metal "compounds" with fixed stoichiometries, and those can vary quite a bit in whole number ratios. However, most inter metallic species are alloys, and may well not have any fixed stoichiometry, so a very large number of bimetallic phases may exist for any pair of metals. This just goes off the scale when one allows polymetal mixes. And if it turns out that superconducting in conductors has analogy to semiconducting in insulators, very small amounts of a metal acting as the superconducting equivalent equivalent of a hole or an electron my dominate the behavior of the material. The analogy may not be a good one, but if it is in any way apposite, the amount of material space to be searched out there is truly gigantic.

You mean those of us who have thrown off the bonds of metric servitude and embraced the one true unit: the hogshead

You mean I have to give up cubits?

Looks like I picked the wrong week to try and give up cubits.

Quantize them to Q-bits and you are a revolutionary rather than a reactionary. But you can remain classical in your heart, and nobody will ever be able to prove that you are wedded to cubits, furlong, long time.

Also, the easier units conversion is one GPa == 10,000 atmospheres (to within about 1%). So, 100 GPa is a megabar.

A priest, a monk and Chris Lee are having a drink. The priest says "God must be disappointed in me, I feel the pressure upon me is too great to handle". The monk says "I know how you feel, however I believe the pressure comes from within, overloading ourselves with the pressures of the world". Chris Lee looks at both of them and goes "yeah... Guys? It's because we came to a megabar".

I still find it amazing that two gases can be combined to form a liquid, H2O!.

You find that amazing? A single gas can be compressed to form a liquid.

Well, the amazing aspect intended by the OP is surely that these two elements who are gasses at room temp and pressure turns into a liquid at room temperature and pressure when combined. Sure either of them by themselves can be turned into liquids but that’s by increasing pressure or reducing temperature. When combined, however, they just do that by themselves. Obv, I get this is how physics and/or chemistry works but it’s still fascinating.

I would prefer if they or some other team tested their calculation as well. Calculations and predictions occasionally disagree wildly with reality. Extreme pressure aside a critical temperature of 350K is extraordinary, so it requires extraordinary evidence, not just a prediction. So will hydrides turn out to be hiding a member among their large group with room temperature and room pressure (or, alternatively, able to retain its high pressure state when the pressure is released, just like diamond) superconductivity? Will they succeed where metallic hydrogen apparently cannot?

I would prefer if they or some other team tested their calculation as well. Calculations and predictions occasionally disagree wildly with reality. Extreme pressure aside a critical temperature of 350K is extraordinary, so it requires extraordinary evidence, not just a prediction. So will hydrides turn out to be hiding a member among their large group with room temperature and room pressure (or, alternatively, able to retain its high pressure state when the pressure is released, just like diamond) superconductivity? Will they succeed where metallic hydrogen apparently cannot?

It's not the only Hydride that work is being done on. lanthanum hydride has been confirmed as a superconductor at 250 Kelvin at 170 GPa. https://www.nature.com/articles/s41586-019-1201-8 . It's unlikely that a Hydride will retain it's superconducting lattice without the pressure.

There are distinct metal "compounds" with fixed stoichiometries, and those can vary quite a bit in whole number ratios. However, most inter metallic species are alloys, and may well not have any fixed stoichiometry, so a very large number of bimetallic phases may exist for any pair of metals.

I'm not sure you are right about "most".There are a relatively small number, IIRC, of straight alloys (intermiscible metals whose phase diagram shows no energy peaks, step changes in dimensions and so on.)There are metals with complex lattices (Pu anybody?) which have intermetallic compounds in which voids can be filled with other metals giving nonstoichiometric ratios. There are others which form a number of different compounds which are stable at the same temperatures, so mixtures can contain an entire range of apparent ratios even though individual grains might consist of a pure compound.

I imagine that for these simulations single crystals - more correctly, regions of constant composition - only are considered, and this immediately reduces the range of possibility. OTOH the downside could be that under real world conditions a material which looks promising may not be obtainable in single crystal form and so would be practically useless. A number of metals can exist as single, fault free (no dislocations, twinning etc.) crystals with very favourable properties, but actually making them in useful quantities is impossible. (Tungsten is something of an exception and its unique metallurgy combined with its high melt temperature explains why incandescent light bulbs lasted so long.)

I still find it amazing that two gases can be combined to form a liquid, H2O!.

You find that amazing? A single gas can be compressed to form a liquid.

Well, the amazing aspect intended by the OP is surely that these two elements who are gasses at room temp and pressure turns into a liquid at room temperature and pressure when combined. Sure either of them by themselves can be turned into liquids but that’s by increasing pressure or reducing temperature. When combined, however, they just do that by themselves. Obv, I get this is how physics and/or chemistry works but it’s still fascinating.

I guess that as someone with a little background in chemistry I find the effects of covalent bonding totally unsurprising, while the fact that you can compress carbon dioxide, thus adding energy, and yet turn it into a liquid, is still remarkable.

Was this based on any experimental evidence or is this being derived from pure theory? If it's the latter, what's stopping them from just running the same calculations on every possible hydride combination?

Yes, it's theoretical:

Quote:

Although 250GPa is too high for practical use—it may even be too high to allow for an experimental demonstration of superconductivity

As for "every possible hydride combination" - there are potentially a lot. Each additional metal atom you add will increase the space by a lot, there are 95 different 1-metal-atom combinations, 9025 2-metal-atom combinations, 857,375 3-metal....and so on. And it could be a case where the equations get significantly more difficult as you increase the number of components.

And we now have a new race with Bitcoin Mining to consume computational (and grid) power. I assume they will continue exploring this route, but I'm guessing it's not trivial to check everything.

That is assuming 1:1 stoichiometries. Once you blow off that assumption, you start to get rather large numbers in a truly bigly way.